In recent years, the photocatalytic activity of titanium oxide has been applied to various environmental depollution techniques including deodorization, prevention of microbial growth, and prevention of fouling. Anatase titanium oxide, which is commonly used as a photocatalyst, has a band gap of about 3.2 eV, and its photocatalytic reaction proceeds by the action of ultraviolet light having a wavelength of about 380 nm or shorter. Therefore, in order to develop its photocatalytic activity, it is necessary for the titanium oxide to be irradiated with ultraviolet light, which results in problems such as a limitation in places where the photocatalyst can be located and in the fields to which it can be applied.
If it becomes possible to use visible light, which plentiful in sunbeams and room lamps, as an energy source to activate a photocatalyst, the reactivity of the photocatalyst will be strengthened, and it will become possible to utilize a photocatalyst in various locations. For this reason, attempts have been made to develop photocatalytic materials capable of being activated by irradiation with visible light.
For example, JP-A 09-262482 (1997) discloses anatase TiO2 having a visible light activity (i.e., capability of being activated by visible light) which is characterized by containing vanadium or chromium incorporated by means of ion implantation. A titanium oxide-based photocatalyst which is made responsive to visible light by incorporating nitrogen in the titanium oxide crystals is reported in JP-A 2001-205103, and one which is made responsive by introducing stable lattice defects into the titanium oxide crystals is reported in WO 00/10706.
The fact that a visible light photocatalytic activity occurs with titanium oxide having oxygen defects (lattice defects in the positions of oxygen) is described in Nippon Kagaku Kaishi 8, pp. 1084-1091 (1986) and JP-A 10-146530 (1998). Oxygen defects can be identified and quantified most effectively by measurement of an ESR (electron spin resonance) spectrum.
A photocatalyst made of titanium oxide which has oxygen defects and is activated by visible light irradiation is reported in Japanese Patent No. 3,252,136. That photocatalyst is characterized by an ESR spectrum in which a signal having a g-value range of 2.003-4 is observed when the spectrum is measured in the dark in a vacuum at 77 K, and the intensity of this signal is increased when the spectrum is measured under irradiation with visible light in a vacuum at 77K.
The titanium oxide photocatalyst reported in JP-A 2001-190953 has an ESR spectrum with at least three peaks in a g-value range of 1.903-2.030 (i.e., the range in which the g value is 1.903-2.030), and the highest peak among these peaks exists in a g-value range of 1.990-2.020.
The presence of foreign metal ions or defects in titanium oxide is effective at making the titanium oxide responsive to visible light, but on the other hand, it worsens the reproducibility of development of photocatalytic activity and may decrease the photocatalytic activity itself, since deactivation of a photocatalytic response which is accompanied by isolation of electrons and holes may originate from such ions and defects. Another problem of the conventional photocatalysts capable of responding to visible light including those which contain nitrogen is that their preparation requires a device for ion implantation or sputtering, and such a device is not suitable for use in mass production.
It is an object of the present invention to provide a photocatalyst which can stably exhibit a high visible light photocatalytic activity, a method for its preparation which is suitable for mass production, a photocatalytic functional product using the photocatalyst, a coating fluid which can be used to manufacture the photocatalytic functional product, and a method of manufacturing the photocatalytic functional product.